Sister chromatid exchange (SCE) analyses, using 5-bromodeoxyuridine (BrdU) dye techniques in adult mammalian tissues, have provided highly sensitive indices of chromosome mutagenesis by mutagens and/or carcinogens. In the present work, we propose to adapt BrdU techniques for cytogenetic studies of SCE formation, and replication kinetics in pre- and post-implantation mouse and/or rat embryonic tissues. Studies will be conducted with selected teratogens.